Semiconductor pressure transducers are frequently used in applications which require operation in harsh environments that are corrosive and/or involve high temperatures. Accordingly, the stress sensing network of transducers used in such applications, must be protected from these harsh environmental conditions in some way in order for the transducer to remain operational over extended periods of time. In the past, surface over-coatings such as silicon nitride, silicon dioxide and the like, have been provided over the stress sensing network to protect it from harsh operating environments. However, such coatings provide only partial protection as the metallized contact areas of the sensing network remain exposed, which presents problems in applications which involve corrosive environments.
One method which enables the transducer to remain operational in corrosive and high temperature environments involves exposing the backside of the transducer to ambient pressure while hermetically sealing the stress sensing network located on the frontside thereof, in the active portion of the diaphragm surface. Such a method is described in co-pending U.S. patent application Ser. No. 08/458,405 filed on Jun. 2, 1995 entitled: HERMETICALLY SEALED TRANSDUCER AND METHODS FOR PRODUCING THE SAME, by A. D. Kurtz and assigned to Kulite Semiconductors Products, the assignee herein. The co-pending application describes a hermetic seal which utilizes a PYROCERAM glass frit or similar means, to bond a cover member to the frontside of the transducer to hermetically seal the sensing network.
The glass must be used in the above-described application to bond the cover member to the inactive portion of the diaphragm because the fabrication processes used in forming the sensing network of the transducer result in raised or lowered surface features depending on whether the transducer has a silicon-on-oxide (silicon-on-silicon) structure or a diffused non-implanted structure. The raised or lowered surface features such as where the sensing network extends into the contact areas, are not planar with the underlying substrate. This makes it virtually impossible to form a hermetic seal by electrostatically or fusion bonding a cover member to the inactive portion of the diaphragm. For instance, in a silicon-on-oxide transducer structure, the entire silicon network and particularly, the regions on the inactive portion of the diaphragm leading to the metalized contact areas (lead-outs) are elevated from the oxide by several microns. Moreover, a finite space is provided between these raised lead-outs on the order of mils to prevent them from electrically shorting together. The step height of the lead-outs and the finite spaces therebetween create gaps between the cover member and the inactive portion of the diaphragm. These gaps make it virtually impossible to form a hermetic seal between the cover and the inactive portion of the diaphragm by electrostatic or fusion bonding and thus, a glass frit must be used to fill the gaps to create a hermetic cover.
Although the glass frit enables the cover member to be bonded to the inactive portion of the diaphragm to hermetically seal the stress sensing network, there are problems associated with this method. For example, the glass frit has a tendency to flow onto the diaphragm during the high temperature sealing process which causes the performance of the sensors to degrade. There are also thermal mismatching issues and other process compatibility issues which require consideration when using a glass frit. All of this makes the glass frit a difficult material to use for fabricating hermetically sealed covers.
It is, therefore, an object of the present invention to provide an improved method for hermetically sealing a semiconductor pressure transducer which substantially eliminates the use of a glass frit to bond a cover member to a transducer.